1. Field of the Invention
The present invention relates to a golf ball, and more specifically to a golf ball comprising a core, and a cover for covering the core.
2. Description of the Related Art
A golf ball used outdoors may likely to cause discoloration with time due to exposure to UV rays (sunlight), wind or rain, or peeling of a coat or crack in the ball may likely to occur due to repeated impacts exerted to the ball. Such discoloration or peeling of the coat is not desirable because it gives an impression that the golf ball is degraded. Various attempts have been made to keep the external appearance of a golf ball good by improving weatherability or durability of the golf ball. For instance, Japanese Unexamined Patent Publication No. 11-216200 discloses a golf ball having improved external appearance with increased whiteness of a cover, wherein a titanium oxide (a white pigment), a red pigment, and a blue pigment are included in respective predetermined contents in a cover composition. Further, Japanese Unexamined Patent Publication No. 2003-52859 proposes a golf ball with improved durability and weatherability, wherein a urethane clear coat is formed as an outermost layer of the golf ball, and an epoxy resin clear coat is formed between the surface of a cover of the golf ball and the urethane clear coat. The epoxy resin clear coat has adhesiveness both to the cover and to the urethane clear coat and is less likely to cause discoloration.
The former publication discloses an art of raising whiteness of a golf ball by optimizing the composition ratio of pigments such as titanium oxide, but does not intend to prevent discoloration of a cover itself. If the resin composing the cover is degraded or causes discoloration by exposure to UV rays or the like, the ball surface shows the tone of the discolored cover because the clear coat formed on the ball surface is colorless and transparent, which makes it difficult or impossible to hide the degradation of the external appearance of the golf ball.
The latter publication aims at raising weatherability and durability of the clear coat, but does not aim at improvement of weatherability of the golf ball itself. Furthermore, in this technology, two coating steps are required: one is to form the epoxy resin clear coat; and the other is to form the urethane clear coat, which is not desirable in the point of simplifying the coating process.
In addition to the above drawback, there is likelihood that photocatalytic action of the titanium oxide to be added as a white pigment may degrade the resin composing the cover, thereby deteriorating weatherability of the golf ball.
Specifically, by exposure to UV rays, an electron-hole pair consisting of a free electron (e−) on a conduction band and a positive hole (p+) on a valance electron band is generated in the titanium oxide due to excitation of the electron on the valance electron band, as represented by the following formula (1).TiO2+hr→p++e−  (1)
The positive hole (p+) is entrapped by OH− in adsorbed water on the surface of the titanium oxide, thereby generating an {dot over (O)}H free radical, as represented by the formula (3). The generated free radical decomposes a resin component by its powerful oxidizing action.H2O→H++OH−  (2)OH−+p+→{dot over (O)}H  (3)
The free electron (e−) reduces Ti4+ in a crystal of the titanium oxide to Ti3+ as represented by the formula (4). Subsequently, O2 in the air is adsorbed to the Ti3+, and turns to O2− as represented by the formula (5). Then, the O2− is reacted with the H+ in the formula (2), and turns to an H{dot over (O)}2 free radical while undergoing the reaction as represented by the formula (6). Similarly to the {dot over (O)}H free radical, the H{dot over (O)}2 free radical decomposes the resin component around the titanium oxide by its powerful oxidizing action.[Ti4+]+e−→[Ti3+]  (4)[Ti3+]+O2→[Ti4+]+O2−  (5)O2−+H+→H{dot over (O)}2  (6)
Normally, it is difficult to completely suppress the photocatalytic action even by inclusion of a UV absorber, a light stabilizer or a like component in the composition of the golf ball, which hinders sufficient improvement of weatherability of the golf ball.